7 Answers
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Fish, fish, fishes: one salmon, one fish, two, two fish; salmon is one fish, haddock another, so two fishes. Works with any noun where the singular and plural are irregularly identical, and the regular plural is used for categories or groups; in a more painfully literal fashion, this also applies to words where the plural has become singular in the common case: one agendum, two agenda; one agenda, two agendas.

"A couple of Jewish people" is a plural; so is "the Jewish people over there". "The Jewish People" is a collective. They're not the same thing, even if the same word is being used.
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byeJul 6 '11 at 21:38

I think I can justify these collective nouns. Person, people, peoples, for example: A person is a person. More than one person, persons. However, "people" is a group, whether all humans ("people") (plural), or a group ("a people", whether national, ethnic, political, etc.). We might call the collection of one of these groups "the people" (plural). So there is the singular "a people", whose plural is "peoples" (or multiple groups), there is the plural term for the collection of a group of "the people", and also there is the plural "people", which is a synonym for "humans". These words are used thus in these numerous senses, complicating the grammar, so correct me if I'm mistaken.

The common plural of person is people. Persons may be common in legalese, but it is very uncommon in day-to-day speech. One normally counts, "one person, two people, three people," etc. When using the word people in this way, the word is not a collective noun, it is a simple plural.

But people is both a plural and a collective -- you can say "two people", but you can also say "a people". That is normally an identifiable national or ethnic group -- and most assuredly not a simple assemblage of more than one person. It's not a collective noun in quite the same sense that flock, herd or murder is; it usually carries ethnic, racial, linguistic, religious or political overtones to the fore. Used in this way, one can talk of "two peoples", "three peoples" and so forth.